The invention is more specifically aimed at mills used in cement making and in the mining industry. These mills consist of a metallic cylindrical shell rotating about its longitudinal axis and containing a grinding charge made up of grinding implements, generally balls, but which may also consist of cylindrical pebbles, ball-shaped pebbles, etc. of varying size. The material to be ground is introduced from one side of the mill and, as it progresses towards the outlet, on the opposite side, it is ground and crushed between the grinding implements.
As the mill rotates, the material to be ground and the grinding charge are lifted up by the lining and, from a given instant, slide downwards again. It therefore follows that the charge of the mill is essentially concentrated in the fourth trigonometric quarter if the mill is rotating in the trigonometric sense, and in the third quarter if the mill is rotating clockwise, and occupies therein, in a radial plane, a "bean"-shaped area as shown in FIG. 4A of Belgian Patent Application 09301481. Grinding is achieved by the shear and frictional forces as the mass moves. To obtain effective grinding and avoid breaking the linings and the grinding implements, it is necessary for the lining to have a profile such that it lifts the charge up as far as a level such that this charge remains compact, because lifting it too much causes balls to be thrown out further than the base of the charge, that is to say that the balls directly strike the lining, without the interposition of material to be ground. From another viewpoint, the charge has to be lifted up high enough for there to be good agitation of the mass. It goes without saying that the fill co-efficient plays a deciding role in the efficiency of the grinding and that the bean-shaped area occupied by the charge has to be of a shape, position and size that are clearly determined in order to achieve effective and optimum grinding.
To encourage the lifting of the material and of the charge, it is known practice for longitudinal undulations which are adapted to the conditions and parameters in which the mill operates to be provided on the plates that form the interior lining of the mill. The problem, however, is that although the undulations or profiles of the grinding plates are adapted to a given mill, this will perhaps no longer be the case if the conditions change. It should in fact be pointed out that the lifting of the charge and of the material depends on a great many factors such as, for example, the size of the mill, its rotational speed, the size of the grinding implements, etc.
Furthermore, the lining plates are subject to intense wear which means that, even if the lifting conditions are optimum at the start, they may rapidly degrade as a result of the change in profile of the plates which is brought about by wear.
Known lining plates also have the handicap of being quite heavy, of the order of 40 to 50 kg, which means that handling them is difficult and dangerous. Reforms in health and safety at work legislation are in any case tending to forbid the handling of objects that weigh more than 25 kg.
Another constraint is that of replacing the worn plates and of attaching them to the shell, especially when they are bolted. In fact it should be pointed out that the shells have, so that the lining plates can be attached, drillings which are arranged generally according to a standard layout, but these standards may vary from one manufacturer to another. Any model of plate which does not suit these standardized drillings would lead to the need to make another drilling in the mill, something which cannot be envisaged because it is an extremely expensive operation which results in holing the shell and an increased risk of leakage of ground material.
Document DE 1126709 describes a mill in which the lining plates form steps in the direction of rotation. Various plates of differing radial depths follow on from one another in a determined order. The face with the greatest radial depth is at the front, when viewed in the direction of rotation. This has the drawback that the grinding implements are thrown, and this reduces the effectiveness of the grinding and increases the risk of the plates becoming broken. Furthermore, the steps of one ring are offset with respect to the steps of an adjacent ring, that is to say that the steps are not axially aligned. This gives rise to shear forces which increase the rate of wear of the plates on the edges.
Document WO 86/04267 also describes a mill in which the lining plates are arranged in steps to form lifting ramps. In this grinder, there is, in fact, just one type of step. Furthermore, the plates are bolted to the shell which means that the layout of the plates is restricted to the configuration of the holes in the shell.
In mills with stepped linings, the inclination of the ramps has to be increased when the diameter of the mill increases. In known mills, this leads to an increase in thickness of all the plates, which increases the total weight of the lining.